Biology 1201A Chapter 20: Chapter-20-Understanding-the-History-of-Life-on-Earth

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Chapter 20 understanding the history of life on earth. Many fossils are found in sedimentary rock. Sediments found in any one place, form distinctive layers that usually differ in colour, mineral composition, particle size, and thickness. If they haven"t been disturbed, the layers are arranged in order in which they formed (youngest on the top) However, these layers have been uplifted, warped, or even inverted by geologic processes. Geologists of the 19th century discovered that the fossils found in a particular sedimentary stratum, no matter where it was found, represent organisms that lived and died at roughly the same time in the past. Since each stratum was formed at a specific time, the sequence of fossils from lowest (oldest) to highest (youngest) strata reveals their relative ages. Geologists used the sequence of strata and their distinctive fossil assemblages to establish the geologic time scale that diagrams the history of life on earth.

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